In my thesis I investigate evolution and diversification of montane birds that inhabit dramatic environmental and latitudinal gradients, specifically within the genus Anairetes (Aves, Tyrannidae). I use phylogenetic, population genetic, and physiological methods to examine patterns of diversification across environmental gradients. In the first chapter, I infer the phylogeny of Anairetes tit-tyrants to provide an essential phylogenetic framework to ask subsequent questions of biogeography and diversification within the group. In the second chapter, I investigated the role of differential adaptation to altitude in promoting diversification and maintaining species limits between A. reguloides and A. nigrocristatus.
The phylogeny of the flycatcher genus Anairetes was previously inferred using short fragments of mitochondrial DNA and parsimony and distance-based methods. The resulting topology spurred taxonomic revision and influenced understanding of Andean biogeography. In the first chapter, I revisit the phylogeny of Anairetes tit-tyrants using more mtDNA characters, seven unlinked loci (three mitochondrial genes, six nuclear loci), more closely related outgroup taxa, partitioned Bayesian analyses, and two coalescent species-tree approaches (Bayesian estimation of species trees, BEST; Bayesian evolutionary analysis by sampling trees, *BEAST). Of these improvements in data and analyses, the fourfold increase in mtDNA characters was both necessary and sufficient to incur a major shift in the topology and near-complete resolution. The species-tree analyses, while theoretically preferable to concatenation or single gene approaches, yielded topologies that were compatible with mtDNA but with weaker statistical resolution at nodes. Previous results that led to taxonomic and biogeographic reappraisal were refuted, and my results support the resurrection of the genus Uromyias as the sister clade to Anairetes. The sister relationship between these two genera corresponds to an ecological dichotomy between a depauperate humid cloud forest clade and a diverse dry-tolerant clade that has diversified along the latitudinal axis of the Andes. Species-tree and the concatenation approaches each reaffirm the use of mtDNA to provide phylogenetic signal for avian phylogenies at the species and subspecies level. This result is due in part to the abundance of informative characters in mtDNA, and in part to its lower effective population size that allows it to track the species tree.
Local environmental pressures can drive the evolution of adaptive traits that confer a fitness advantage to an organism under local conditions. In the second chapter I investigate the role of differential physiological adaptation to altitude between sister-species: the elevationally widespread A. reguloides and the high-elevation restricted A. nigrocristatus. I measure the physiological response of each species to low ambient partial-pressure of oxygen at high elevations. At high elevation, A. reguloides shows evidence of hypoxic stress while A. nigrocristatus shows evidence of hypoxia resistance. I further quantify the phenotypic and genetic cline shape and the rate and direction of gene flow between the two species across a narrow contact zone where hybridization occurs at middle elevations. Phenotypic and genetic clines show a dramatic shift from A. reguloides to A. nigrocristatus across the 212 km elevational transect. Coalescent-based Isolation with Migration (IMa2) analysis suggests effectively zero introgression between parental populations. Upon secondary contact, the two species segregate elevationally. Physiological data, phenotypic and genetic cline shapes, and introgression patterns suggest that maladapted A. reguloides alleles are selected against at high elevations in the presence of hypoxia resistant A. nigrocristatus alleles. Differential local adaptation is associated with restricted gene flow and essential reproductive isolation, despite at least limited reproductive compatibility.